通用信息

名词释义

Electromagnetic radiation is in general a mixture of many small packets of energy. Each packet, also called photon, possesses a certain amount of energy. Each photon behaves like a wave with a definite frequency and wavelength. If the radiation is visible light, the wavelength corresponds to a colour: the wavelengths of 444, 555, and 666 nanometers are blue, green, and red light, respectively. White light contains all colours.

A spectrum is a kind of representation that shows how much of each wavelength is present in a given field of electromagnetic radation. The solar spectrum is an important example. It contains large amounts of visible light with an emission maximum around 500 nanometers. However, it also contains considerable amounts of ultraviolet and infrared radiation.

Electromagnetic radiation is created by the propagation of two coupled oscillating fields, an electric and a magnetic field. The wavelength of a packet of radiation is the distance between two maxima (or minima) of that oscillation. It can be converted into frequency or energy of the radiation. It is usually measured in meters or a derived unit such as nanometers. Visible light was wavelengths between 400 and 700 nanometers.

When light (electromagnetic radation) encounters matter, some fraction of that light can be absorbed by the matter. Each kind of matter (atoms, molecules, crystals, plasmas...) absorb a certain set of wavelengths depending on, for instance, the distribution of electrons or the mobility of particles in the material. When a known spectrum of radiation is sent into a sample, then the spectrum that is detected behind the sample contains information about the composition about the sample. With good calibration, it is possible to obtain very precise quantitative results.

Transmission is the process to make light (electromagnetic radiation) pass through a sample. Commonly, the original spectrum of radiation is compared to the transmitted spectrum in order to identify absorption at certain wavelengths. That absorption pattern contains information about the composition of the sample. Additional reflection or scattering processes need to be considered and can be taken into account by adequate spectral calibration. If the sample is so dense that it is opaque, transmission measurements are not possible. Reflection measurements are an alternative for such cases.

Reflection occurs when light (electromagnetic radiation) hits a sample and is directed back instead of entering the sample. Reflection can be diffuse (evenly distributed in all directions, think of sand) or specular (angle of reflection equals angle of incidence). Most common, however, are intermediate forms - think of the vague reflection of the sun on an otherwise matte surface). Reflection depends on wavelength, and in general certain wavelengths (not necessarily visible) are still capable of penetrating the probe and are subject to absorption. Therefore, reflection measurements can also be used to investigate the composition of a sample.